Silver polish



United This invention relates to an improved silver polish and more particularly to a silver polish containing as an essential ingredient at least one di-substituted lower alkyl thiourea.

In the prior art, many silver polish formulations have been proposed in an attempt to repress the tendency of surfaces of silver to tarnish and stain upon exposure to ordinary atmospheric conditions. These characteristics present an annoying and time consuming cleaning problem for the housewife. Tarnish, as the term is used in this specification, embraces a film discoloration of the silver surface which occurs as a result of a chemical change in the metal as opposed to a mere physical soiling. For example, the formation of a sulfide or oxide film, which is from all indications, integral with the silver surface and similar to metal corrosion.

As usually made, the prior art silver polishes comprise an abrasive material and a carrier such as Water, a hydrocarbon oil or glycerine, either with or without a soap or an emulsifying agent. Silver polishes are also known which contemplate the use of film forming materials in polishes such as waxes and resins and in addition, the use of anti-tarnish components such as the mercaptans for the prevention of re-tarnishing. The use of sulfur compounds in silver polishes and cleaners was originally looked upon with apprehension and received only hesitant acceptance by the manufacturers of silver articles because sulfurous compounds such as H 8 in the atmosphere were known to be a primary cause of silver tarnish. Thus, the use of a sulfur bearing material to prevent silver tarnish was thought to be anomalous.

However, the use of sulfurous materials has now received acceptance and US. Patent 2,628,199 is directed to the use of thiourea in an acidic dip-type silver and copper cleaning composition. US Patent 2,691,593 contemplates a silver cleaning composition in paste or liquid form containing abrasives, an emulsifying agent, a hydrocarbon carrier and if desired, a mercaptan or thiourea. US. Patent 2,841,501 embraces a silver polish containing a mild abrasive and a long chain alkyl mercaptan as an anti-tarn-ishing agent. Allegedly, the mercaptan forms a thin protective film upon the silver and prevents re-tarnishing.

These silver cleaners and polishes are of substantial value; however, each has disadvantages and limitations. For example, it has been found that acidic dip-type formulations have a deleterious elfect upon silverware, possibly resulting in more rapid re-tarnishing after the silver is again exposed to normal use and possibly causes removal of the silver surface. The composition apparently functions primarily as a cleaner and provides little or no protective coating. On the other hand, polishing compositions similar to those described in Patent No. 2,691,593, because of the hydrophobic earlier employed, are relatively difficult to wash off the silver after polishing and there is an indication that the silver tarnishes more easily after the initial cleaning. The rapid re-tarnishing may be a result of more exposed surface area due to scratches on the silver surface.

Thus, it is an object of the instant invention to provide a silver polishing composition which ww'll rapidly remove tarnish from silver without harming the surface of the article.

"atent O It is another object of the invention to provide a p01- ishing composition which will protect the cleaned silver surface against rte-tarnishing by the deposition of a thin transparent film on said surface.

It is still another object of the invention to provide a polishing composition which will quickly remove tarnish from the silver surface without leaving noticeable scratches upon the silver surface.

It is still another object of the invention to provide a silver polishing composition, alkaline in nature, which does not separate upon storage and thus, has a long shelf life.

These and other objects of the invention will become more apparent from the following detailed description with particular reference to the illustrative examples.

According to the instant invention, an improved aqueous silver polish is formulated which contains as an essential ingredient at least one di-substitu'ted lower alkyl thiourea, and preferably a member of the group consisting of dimethyl thiourea, diethyl thiourea, diisopropyl thiourea and dibutyl thioure'a. These substituted thioureas are found to have surprising and unobvious properties in silver polish formulations which contain an abrasive, preferably in an aqueous alkaline carrier. The above di-substituted thioureas are extremely effective cleaning and anti-tarnish components. Apparently, the (ii-substituted thioureas form a thin protective film on the clean surface of the silver, possibly bonded to the surface by a secondary chemical force. The protective film remains on the silver surface for a substantial period of time. However, while providing protection against retarnishing, the transparent protective film is still thin enough so as to be unnoticeable on the silver thus, leaving the esthetic appearance of the silver unimpaired.

In order to more explicitly demonstrate the unexpected and unobvious properties of the instant silver polish-es, four embodiments of applicants invention, Examples 1 through 4, containing dimethyl, cliethyl, di-isopropyl and dibiutyl thiourea, respectively, were compared with formulations having various modifications. Example 5 illustrates the basic formulation employed in Example 1 omitting the thioureas; Example 6 employs simple thiourea. From visual observations made on silver spoons polished with the several formulations the superiority of those employing di-substituted thioureas, in their effect upon re-tarnishing of the silver upon exposure to contamina tion conditions, Was vividly demonstrated.

Example 1 I. 35 parts by weight abrasive clay, at least of which has a particle size of 5 microns or less in diameter, and not more than 0.01% of the particles remaining on a 300 mesh screen, and a hardness of between 8 and 9 on a scale wherein diamonds have a hardness of 10and 0.10 part Krorna Red, a ferric oxide pigment were stirred to give a uniform dry mix.

11. A premix of 7.5 parts of water and 1.5 parts soda ash was made at room temperature.

III. 40.55 parts water were heated to a temperature of F. and 3.0 parts propylene glycol, 0.2 part Versene 100, a concentrated, straw colored, aqueous solution of the tetrasodium salt of ethylenediaminetetraacetic acid, 0.10 part thiourea, and 1.5 parts dimethyl thiourea were added to the heated water. Thereafter, 9 parts Amber Flakes, a soap powder having a real soap content of 88% with the remaining 12% being made up of moisture, rancidity inhibitors and ash, were added with agitation to provide a smooth, flowable liquid. The temperature of the mixture was approximately 123 F.

IV. The soap solution of part III was added with constant stirring to the dry mix I to obtain a thoroughly mixed flowing paste. During the stirring, it was necessary to exclude, as much as possible, the incorporation of air into the mixture.

V. The mixture IV was added to premixed II, above, with constant agitation. The total mixture was a relatively thick paste.

VI. To the paste V was added 1.5 parts Hyonic FS, a lauric acid alkanolarnide surfactant. The composition was stirred to obtain a creamy homogeneous paste at room temperature. The product flowed when heated to a temperature of 110 F.

Example 2 A polish was prepared using the procedure and in gredients as in Example 1 except that diethyl thiourea was substituted for the dirnethyl thiourea. The polish was a creamy homogeneous paste at room temperature.

Example 3 A polish was prepared using the procedure and ingredients as in Example 1 except that diisopropyl thiourea was substituted for dimethyl thiourea in part III of the formulation. The polish was a creamy homogeneous paste at room temperature.

Example 4 A polish was prepared using the procedure and ingredients as in Example 1 except that dibutyl thiourea was substituted for dimethyl thiourea in part III of the formulation. The polish was a creamy homogeneous paste at room temperature.

Example 5 Another polish was prepared using the procedure and ingredients as in Example 1 except for the omission of the thiourea and di-substituted thiourea in part III of the formulation. The polish was a creamy homogeneous paste at room temperature.

Example 6 A polish was prepared using the procedure and ingredients as in Example 1 except for the omission of the di-substituted thiourea in part III of the formulation. The polish was a creamy homogeneous paste at room temperature.

The formulations in Examples l6 were applied uniformly to two types of silver exercising every precaution to insure, as nearly as possible, identical application. Two types of silver spoons were selected since experience has shown that different silverwares react somewhat difierently in their tarnishing characteristics. One type sterling silver used was design free and the second type had an Old Lace style pattern. The spoons were numbered with a diamond pencil for identification purposes. The spoons were then cleaned with an abrasive and Water using a sponge applicator to insure complete removal of any trace of chemical from past experiments which may have remained on the spoons.

The spoons were cleaned by placing approximately 2 grams of the test polish on a damp sponge applicator and rubbing over the spoon. The spoon and sponge were rinsed and the cleaning operation repeated a second time. The spoons were again rinsed with a amount of water and dried in one instance and in the second case the polishing material was allowed to dry on the spoon and thereafter polished with a dry soft cloth. The spoons were then placed in a test tube rack and the entire rack and spoons placed in a bell jar. Hydrogen sulfide was passed into the jar at a very slow rate, determined by a bubble indicator. The spoons remained in the jar for a period of 45-60 minutes at which point they were removed.

The spoons were examined visually in a good source of natural light. The results were as tabulated:

Example l-very slight tarnish; difficult to observe Example 2-very slight tarnish; difficult to observe Example 3slight tarnish; difficult to observe Example 4s]ight tarnish; difiicult to observe Example 5rnoderate tarnish Example 6-heavy tarnish A definite pattern was established in repeated tests, clearly showing that the most severely tarnished spoons were those cleaned with the polish of Example 6 which contained only simple thiourea. The next most severely tarnished spoons were those cleaned with the polish of Example 5, the basic soap formula. In the remaining spoons tested, i.e., those polished with formulations containing the di-substituted thioureas, the differences in tarnish were difiicult to observe. However, at times, there was a visual difference between the amount of tarnish deposited on silver cleaned with the polishes of Examples 1 and 2 and on the silver cleaned with the polishes of Examples 3 and 4. It is apparent that the polishes utilizing the lower alkyl disubstituted thioureas in the soap formulations are substantially and surprisingly superior to the examples which did not contain the (ii-substituted thioureas. This difference could not be predicted from the prior art or what one would expect from closely related members of a homologous series.

To summarize therefore, the simple soap formulation offered little or no protection against silver tarnish. When thiourea alone was added to the soap formulation, the tarnishing effect was increased. When using a lower alkyl di-substituted thiourea in the formulation, the tarnishing, surprisingly, was inhibited with no or only very slight tarnishing. These results were completely unobvi ous and unpredictable.

In its basic form, the silver polish of the present invention comprises as essential ingredients a di-lower alkyl substituted thiourea, and preferably at least one member of the group consisting of dimethyl thioreau, diethyl thiourea, diisopropyl thiourea and dibutyl thiourea, an abrasive, and preferably an aqueous alkaline carrier. As is apparent from the discussion hereinbefore, the di-substituted thiourea functions primarily as a tarnish inhibitor. These materials cannot be replaced by other materials and must be present in the formulation.

An abrasive is also necessary in the instant formulation in order to obtain effective and rapid cleaning of tarnished silver. Substantially any abrasive may be used in the polish as for example, charcoal, chalk, rouge, tripoli, pumice, brick, pipe clay, yellow ocher, emery, rotten stone, silica, sand, flint, infusorial earth, red lead, white lead and the like. These abrasives are water insoluble. Although any of the above abrasives can be used effectively in the instant silver polishes, surprisingly advantageous results were obtained in formulations employing a clay having an average particle size of approximately 1.4 microns in diameter and with less than 5% of the clay having a particle size of more than 5 microns in diameter. In addition to fine particle size, the clay should be extremely hard, as for example, a hardness of between 8 and 9 on a scale wherein diamonds have a rating of 10.

It was further found that where the clay particles are composed of thin flat sheets they are more effective. Be cause of the extremely fine particle size of the clay, it does not appear to scratch the surface of the silver. However, it is apparent that the silver surface is scratched, but that the scratches are so minute that they will not refract visible light and accordingly, cannot be seen. The extreme hardness, on the other hand, provides good cutting action and cleans the tarnished silver quickly. Moreover, since the clay particles are composed of thin flat sheets, the cleaning action is eifective with only a limited amount of scratching by the abrasive and thus good cleaning is achieved with a minimum removal of silver.

It has also been found that improved results are obtained with the instant silver polish if an aqueous alkaine carrier is employed. When such a carrier is used, the cleaning action is extremely rapid and moreover, the

polish can be easily washed off after cleaning, whereas if an oil or grease carrier is employed, difficulty is experienced in washing unless a special solvent or emulsifying agent is employed. The alkaline carrier can be made by dissolving sodium hydroxide, borax, ammonia, sodium bicarbonate or similar basic materials in water. Preferably the pH is in the range of from about 8 to 11. However, other carriers such as alcohols and hydrocarbon oils may be employed.

The instant polish formulations preferably contain, in addition to the abrasive and substituted thiourea, other ingredients which may conveniently be referred to as additives which assist in the removal of tarnish on the metal surface, or in producing a homogeneous paste. Such materials are surfactants, such as the lauric acid alkylol amide employed in the above examples. However, as a practical matter, virtually any other surfactant may be used so long as it promotes foaming and rinsability in the silver polish. Examples of suitable surfactants are the non-ionics and anionics including the alkyl sulfates, modified sodium alkyl sulfates, nonionic coconut fatty acid alkanolamine condensates, the nonionic fatty amide condensate fatty esters and salts of fatty alcohol sulfates, and the polyoxethylene sorbitol laurate and polyoxyethylene sorbitol oleatelaurates. However, a surfactant is not completely essential in the instant formulations and if desired, a suitable choice is within the ability of one skilled in the art.

It is also desirable to include a humectant in the silver formulation such as propylene glycol, glyccrine, ethylene glycol, or other known humectants. Metal ion complexing agents such as the tetrasodium salt of ethylene diamine tetraacetic acids may also be desirable to prevent reversion of the soap base formula during storage or while it is on the shelf of a supermarket, etc. Further, while not completely essential, a soap flake material in the formulation will improve the cleaning action. Any soap flake can be employed as long as it is composed pri marily of real soap and contains a rancidity inhibitor. It is essential that the soap contain a rancidity inhibitor to prevent souring upon storage for prolonged periods of time on supermarket shelves.

Finally, it may be desirable to include a pigment and a perfume in the instant silver polish formulation for consumer appeal. The pigment, of course, is merely present to add color to improve the appearance of the formulation, thus, any color can be used which is compatible with the other ingredients in the polish. The proper choice of a pigment is within the ability of one skilled in the art. The perfume is also non-essential to the cleaning and anti-tarnishing characteristics of the polish, however, as is true with most silver polishes, during the polishing operation, a sour odor is given off, and therefore, it is desirable to include a perfume which will cover odors developed during the cleaning of the silverware and to promote its general public acceptance. It was found that a complex mixture of aldehyde, lactones, esters, ethers, ketones and essential oils was particularly effective in masking the odors evolved.

The proportions and methods of mixing the ingredients of the silver polish are not particularly critical, there being a fairly wide range of operable proportions depending primarily upon whether the preparation is to be used as a cream or paste. Usually, the formulation is most conveniently used as a fairly viscous cream. The base formulation is composed of from about 30-80% carrier, 20-70% abrasive and 0.5-3.0% diethyl, dimethyl, diisopropyl or dibutyl thiourea. Although the above range of the materials provides operable silver polishes, a preferred range is from about 40-70% carrier, 30-60% abrasive and 1-1.5% disubstituted thiourea. All percentages are by weight.

Additional ingredients in the silver polish can be dispensed with and reasonably satisfactory results are obtained; however, a more effective product, at least so far as consumer acceptance is concerned, is achieved if additives are present in approximately the following range of proportions by weight, replacing proportionate amounts of base materials:

Percent Surfactant (as for example, lauric acid alkylol amide) 1 to 3.0 Humectant (propylene glycol) 1 to 10 Metal complexing agent (tetrasodium salt of ethylene diamine tetraacetic acid) .01 to 1.0 Soap 6 to 12 Pigment .05 to 1 Perfume .03 to 1 Alkaline material (soda ash) 0.5 to 5 As is apparent, therefore, a proper selection of the nonessential ingredients and the adjustment of the ratio of the base materials to include these materials is within the ability of one skilled in the art.

The invention has been described with reference to particular embodiments and examples. However, it is apparent that variations and modifications may be made and that equivalents can be substituted without departing from the true spirit of the invention. These modifications are to be included within the scope of the invention.

What is claimed is:

1. An improved silver polish formulation consisting essentially of the following materials in substantially the amounts stated:

Percent Water 48.05 Sodium carbonate 1.50 Lauric acid alkylolamide 1.50 Propylene glycol 3.00

At least one member of the group consisting of diethyl thiourea, dimethyl thiourea, diisopropyl thiourea, and dibutyl thiourea 1.50 Tetrasodium salt of ethylene diamine tetraacetic acid 0.20 Soap 9.00 Abrasive 35.00 Thiourea 0.10 Pigment 0.10 Perfume 0.05

2. An improved silver polish formulation consisting essentially of from about 40 to 70% water, about 30 to 60% abrasive, about 1 to 1.5% of a member of the group consisting of diethyl thiourea, dimethyl thiourea, diisopropyl thiourea, dibutyl thiourea and mixtures thereof, 1 to 10% humectant, .01 to 1.0% polyvalent metal chelating agent, 6 to 12% soap, .05 to 1.0% thiourea, and 0.5 to 5% of a compatible alkaline material.

3. An improved silver polish formulation comprising as essential ingredients from about 30 to of a liquid carrier selected from the group consisting of aqueous alkaline, alcohols, and hydrocarbon oils, about 20 to 70% abrasive, and 0.5 to 3.0% of a member of the group consisting of diethyl thiourea, dimethyl thiourea, diisopropyl thiourea, dibutyl thiourea, and mixtures thereof.

4. The formulation of claim 3 employing an aqueous alkaline carrier having a pH in the range of from about 8 to 11.

5. The formulation of claim 4 wherein at least of the abrasive has a particle size of not over 5 microns and a hardness of between 8 and 9 on a scale wherein diamonds have a rating of 10.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN IMPROVED SILVER POLISH FORMULATION CONSISTING ESSENTIALLY OF THE FOLLOWING MATERIALS IN SUBSTANTIALLY THE AMOUNTS STATED: 